Since the early reports, see the publication "Properties and Device Applications of Magnetic Domains in Orthoferrites", A. H. Bobeck, The Bell System Technical Journal, October 1967, pages 1901-1925, intensive studies have been conducted on cylindrical domains, single wall domains, bubble domains or more simply bubbles, in magnetizable films with perpendicular anisotropy. The chief intended application is as a solid-state memory element replacement for disc files. The economic criteria for a viable bubble domain memory technology are therefore well defined in the commercial marketplace, the chief criterion being low cost. For military and space applications, additional criteria become evident. The most obvious approach to meeting these criteria is an increased device density by reducing component size of conventional-design bubble devices. The state of the art bubble devices utilize 5 micron (.mu.m) diameter bubbles and a Permalloy-bar structure--see the publication "Magnetic Bubbles", A. H. Bobeck, et al., Scientific American, September 1970, pages 78-90. This type of device can be useable with bubbles down to a 3 micron diameter when photolithography is used. Further increase of density in this type of structure requires the use of e-beam mask generation and x-ray resist exposure. Submicron-size bubble devices have been fabricated by this means--see the publication "Bubble Device Overlay Fabrication Using Scanned Electron Beams", D. Webb, Microelectronics, Volume 7, No. 1, 1975, pages 22-26. When working with e-beam/x-ray processes it becomes quite apparent that tolerances are difficult to hold, that the implied larger number of bits per chip decreases device yield, and that generally the newer processes are more costly and difficult to use as compared to photolithography, especially for multi-layer devices.
Several concepts have been proposed to allow erasing of constraints on lithographic tolerances--see the publication "Magnetic Bubbles--An Emerging New Memory Technology", A. H. Bobeck, et al., Proceedings of the IEEE, Volume 63, No. 8, August 1975, pages 1176-1195. One concept is the contiguous disc file. This concept utilizes magnetic features that are large compared to bubble diameter, and, as a result, for a given lithographic resolution, a four-fold increase in device density may be achieved. This type of concept has a disadvantage that discrete features are required to define the discrete storage cells. A second concept intended to increase bubble density is the bubble lattice file (BLF)--see the publication "The Use of Bubble Lattices For Information Storage", O. Voegeli, et al., AIP Conference Proceedings, No. 24, pages 617-619, 1975. The BLF eliminates potential-well structuring features and uses wall structures for information storage. The BLF suggests that the elimination of discrete features for each storage cell is a key conceptual design factor for high density bubble devices. The BLF, as presently conceived, still suffers one serious drawback; that is the inability to propagate a long series of bubbles without complex propagate circuitry. This fact leads to a second conceptual design factor for design of high density devices; elimination of complex propagate circuitry. It cannot be expected that all discrete structuring and propagate features must be eliminated. However, it would suffice if the storage areas only could be designed to use simplified structuring and propagate schemes since, as the devices become larger, the storage role predominates over the special functions of write/address/read.
U.S. Pat. No. 3,940,750 describes a concept wherein information in the form of data bits is stored as polarity reversals in linear domain walls separating adjacent magnetic domains. While offering high storage density, realization of the concept requires use of combinations of mechanisms of unproven reliability. The present invention is directed toward a concept that utilizes a simplified structuring and propagate scheme.